The present invention relates to apparatus for remote signaling to a telephone line over the available AC power line. More particularly, the present invention provides a telephone extension system for communicating over the AC power line between telephones of the system and a conventional telephone subscriber telephone line and for intercom communication between telephones of the system.
U.S. Pat. No. 3,949,172 which issued April 6, 1976, entitled "Telephone Extension System Utilizing Power Line Carrier Signals", which is assigned to the same assignee as the present application, describes a system for doing some of the same thing as the system described in the present application. That patent describes a main (or master) station that couples the subscriber's telephone line to his AC power line and a special extension telephone that plugs into his AC power line at any convenient power receptacle and is used to make and receive telephone calls in a conventional manner. In that patent, the extension telephone includes a hand set with a microphone at one end and an earphone at the other with separate lines from each. The line from the microphone feeds a radio frequency (RF) transmitter and the line to the earphone comes from an RF receiver. A corresponding RF transmitter and RF receiver are provided at the main station. These transmitter-receiver combinations enable full duplex signalling of voice and other signals between the main station and the extension telephone over the AC power line.
The power line, RF carrier extension telephone system in U.S. Pat. No. 3,949,172 provides for a main (or master) station and one extension telephone that plugs into the AC power line at a power line receptacle. That system can accommodate more than one extension telephone and if all transmit at the same extension RF carrier frequency, the main can respond to any one of them. However, the single receiver at the main can respond to only one extension carrier at a time and so the extension telephones of the system cannot conference with each other full duplex on a telephone call. Full duplex conferencing between such extension telephones is achieved in the system described in U.S. Pat. No. 4,475,193 entitled: A Power Line Carrier Multi Telephone Extension System For Full Duplex Conferencing Between Telephones; by William M. Brown (an inventor in the present application) and also assigned to the same assignee as the present application. It is an object of the present invention to provide such a power line carrier telephone extension system including several extension telephones that all can operate at the same time and engage in full duplex communication with each other without going on the telephone line (Intercom), while at the same time, any of the system telephones not engaged in such intercom communication can place or receive calls from the telephone line.
It is another object to provide means for signalling to the system telephones while they are engaged in intercom communication, that an outside telephone call is received so that any or all of those telephones can answer the outside call.
It is another object to provide a multi-telephone extension system for a telephone subscriber's premesis capable of Intercom and conferencing between the system phones and capable of other functions, such as Hold, Privacy and Special functions, as they are defined herein, over a two element intra-premises transmission line between phones of the system.